This application relates to control of contaminating particles in materials processing.
In a sputter or reactive ion etching or chemical vapor deposition apparatus used in semiconductor manufacturing, a semiconductor wafer is placed between two electrodes which are driven by a radio frequency (RF) AC voltage source. (Ordinarily the wafer is placed directly over one electrode and a substantial distance from the other electrode.) A reactant gas is released into the chamber in the area above the wafer. Because the reactant molecules are relatively large and immobile, they cannot move in response to the rapidly fluctuating electric fields. However, the electrons associated with the reactant molecules are relatively mobile. As a result, if the AC electric fields are sufficiently intense, electrons are stripped from the reactant molecules, forming a gas plasma of free electrons and positive reactant ions. This plasma is highly conductive because it contains a large number of free carriers, and therefore is essentially an equipotential region.
When the RF AC source is first turned on, the wafer and electrode surfaces contain no charge. During the first half of the AC cycle when the electrode beneath the wafer has a negative voltage, the negative voltage causes positive ions from the plasma to strike the wafer surface, removing electrons and leaving behind a positive charge on the surface. During the second half of the AC cycle, the electrode beneath the wafer has a positive voltage, and the wafer surface attracts and collects electrons. The mass of the electrons is much smaller than that of ions, and the electric field accelerates them more rapidly to the electrode surface. As a result, after one AC cycle, many more electrons are collected on the wafer surface than ions. As more cycles transpire, the wafer surface continues to build negative charge.
This accumulation of negative charge causes the wafer to develop a negative DC voltage relative to the plasma. As a result, during a larger part of the AC cycle the wafer voltage is more negative than the plasma, reducing the time during which electrons are attracted to the wafer and increasing the time during which positive ions are attracted to the wafer. Ultimately, the wafer charges to a negative voltage nearly equal to the peak voltage of the AC signal, and the system reaches a steady state in which an equal number of positive ions and electrons strike the electrode during each complete signal period. The flow of ions is relatively steady throughout the cycle, and is equalized by a brief spike of electron flow during the peak portion of the AC cycle. The negative steady-state DC offset voltage between the wafer and plasma is known as the "self bias voltage".
Because the wafer is charged to a negative self bias voltage, ions which reach the edge of the region between the plasma above the wafer (the "plasma sheath") are accelerated onto the wafer surface and come to rest within the semiconductor lattice of the wafer surface. Since the plasma is an equipotential region, ions within the plasma are not influenced by the negative voltage of the wafer; only those ions which happen to reach the sheath boundary due to random motion are accelerated into the wafer. Thus the ion flow mechanism is diffusive in nature.
Particle contamination is becoming an increasingly serious limitation in high quality materials processing such as semiconductor manufacturing. In the semiconductor manufacturing area, it has been estimated that as much as 70-80% of all wafer contamination is caused by particles. Thus, substantial reductions in defect rates may be achieved by reductions in particle contamination.
A typical plasma apparatus includes many potential sources of contaminating particles, such as: cracked or flaking materials (e.g., quartz) or films (e.g., dielectric films) inside the chamber, polymers collecting on the walls of the chamber over time, small metal spheres created by arcing between metal surfaces, and incidental contact or rubbing inside the chamber during wafer handling. Once particles from any of these sources are released into the processing chamber during processing, they enter the gas plasma, and ultimately land on and contaminate the wafer surface.
Because free electrons within the plasma have a much higher mobility than positive ions, any materials floating within the plasma, e.g. particles, are bombarded with more electrons than ions, and will build up a negative surface charge.
Gravitational forces pull the particles in the plasma down in the direction of the wafer; however, when the RF AC source is engaged during the plasma process, this gravitational force is equalized by a repulsive electrostatic force between the negative charges accumulated on the wafer and particle surfaces. As a result, as shown in FIG. 1A, particles 10 float above the wafer 12 during the plasma process. (The total charge accumulated on a particle is roughly proportional to the surface area and mass of the particle, as a result, large particles and small particles both tend to float approximately the same distance above the surface of the wafer.) Selwyn, "Particle trapping phenomena in radio frequency plasmas" (Applied Physics Letter 57, Oct. 29, 1990) demonstrated that a typical wafer chamber generates (electric field) equipotential wells above the wafer which trap particles above the wafer. Selwyn photographed particles floating above wafers using scattered laser light.
Although the balance of gravitational and electrostatic forces prevents particle contamination while the RF power is on, after the RF power is turned off, the negative charge on the wafer dissipates, and, as shown in FIG. 1B, as a result the particles 10 fall onto and contaminate the wafer 12.
Selwyn, "Plasma particulate contamination control II. Self-cleaning tool design" (Journal of Vacuum Science and Technology, 10(4), July/August 1992) discusses this effect and suggests that the wafer electrode can be grooved to create channels in the equipotential well leading away from the wafer, allowing particles to flow out away from the wafer during plasma processing.